In the solution ring opening polymerization of cycloolefins, the product discharged from the reactor is a honey-like cement that consists principally of a cyclic nonpolar carrier solvent in which the polymer is dissolved. The polymer content is normally on the order of about 15% by weight. The polymer can be any of the family of polymers that are made by homopolymerization or copolymerization of one or more of cycloolefins that contain the norbornene group. Polymerization can be either batch or continuous.
After the honey-like cement is prepared, it is necessary to separate the polymer from its carrier solvent. In the past, steam stripping has been used exclusively in plant operations to extract the polymer from the carrier solvent. In steam stripping, the cement is injected into a jet of steam that is directed into a vessel containing hot water. As contact is made between the cement and the jet of steam, the carrier solvent is flashed off as vapor, depositing the polymers in particle form in the hot water.
Steam stripping has a number of serious disadvantages. It produces a product of relatively large, coarse and variable particle size. The product contains a substantial amount of occluded water that makes it extremely difficult to dry. It produces a product that retains significant quantities of residual high boiling monomers and other residues that include high boiling reaction products and catalyst residue, all of which adversely affect the quality of the final product. Steam stripping has the inherent carry-over problem of polymer fines with the solvent vapor and steam that is continuously discharged. This carry-over results in severe plugging in the solvent recovery system. Finally, steam stripping requires large volumes of steam, far in excess of that required to vaporize the solvent in order to produce a particulate product. Steam stripping, therefore, is an inefficient, expensive, and energy-wasteful operation.
It has been a common practice to reduce impurities in the polymers of cycloolefins by water washing the cements before steam stripping. In this way, effects of certain water-sensitive impurities are eliminated. Water washing has not been practical since large volumes of contaminated water were produced that had to be disposed without creating an environmental problem.
More recently, an alternate approach was discovered for isolating polymers of cycloolefins from the carrier solvent. Pursuant to this approach, the cement is mixed in a high shear mixer with a nonsolvent in the volume ratio of about 3 to 1 nonsolvent to cement whereby the polymer precipitates out. A nonsolvent is a liquid that is miscible with the nonpolar solvent that is used in the polymerization reaction but is a nonsolvent for the polymer. Examples of suitable nonsolvents include, ethanol, propanol, isopropanol, and the like. Although on some occasions this recovery procedure produced granular, easy-to-dry product having bulk density of about 0.144 g/cc or 9 lb/ft.sup.3, these results could not be reliably reproduced. What was obtained normally was a clump-like product of fine, irregular fluffy microfibers that packed cotton-like when filtered and was difficult to dry and handle, the dry product having bulk density below 0.08 g/cc or 5 lb/ft.sup.3.
When polymer cement is precipitated or coagulated in a nonsolvent medium, the high polymers appear to precipitate from and the oligomers, catalyst residues, and the like, remain solubilized in the mixed nonsolvent-solvent medium. Since a substantial portion of the impurities are soluble in the nonsolvent, this recovery process succeeded in removing the bulk of the impurities from the polymer. However, this approach was not entirely successful since large volumes of contaminated liquid was produced composed primarily of nonsolvent, cyclic nonpolar reaction solvent, and impurities that included residual shortstop for the polymerization reaction, adducts of the shortstop with catalyst residues, residual catalyst components, oligomers, etc. Solvent recovery of the large volume of nonsolvent--solvent liquid is difficult and expensive and especially complicated using water-free nonsolvents which form azeotropes with water and the solvent.
More specifically, in reference to the use of a nonsolvent in polymer extraction, at bottom of col. 4 of the Minchak U.S. Pat. No. 4,069,376 it is disclosed that a polymer of one or more cycloolefins can be isolated by precipitation using a nonsolvent selected from lower alcohols such as methanol, ethanol, isopropanol, and the like. This is a known method wherein polymer cement and a nonsolvent are mixed in a high-shear intensive mixer whereby a slurry is formed. The slurry is then conveyed to a slurry tank where it is further agitated at ambient temperature, and from the slurry tank the slurry is taken to a centrifuge or a filter where the polymer is separated and taken to a drying operation whereas the filtrate is pumped to recovery where the cyclic nonpolar solvent is separated from the nonsolvent and recovered. The filtrate is composed essentially of the cyclic nonpolar solvent and the nonsolvent hydrocarbon.
Recovery of the cyclic nonpolar solvent, modifier and the water-free nonsolvent is complicated, requiring extraction and distillation operations, as illustrated in FIG. 1 where stream 10 enters extraction vessel 12 at the lower portion thereof and is countercurrently extracted by water stream 14 that enters from the top. Extraction efficiency may be improved substantially using a Podbielniak.RTM. centrifugal extractor or other similar mechanical extraction devices. Stream 10 is composed of the filtrate stream from the centrifuge in the polymer extraction process as well as a ternary azeotrope. For purposes of illustration, cyclohexane is used herein as an example of a suitable cyclic nonpolar solvent, 1-hexene as the modifier, and ethanol is used as an example of a suitable nonsolvent. Stream 16, composed of cyclohexane, 1-hexene, and a trace of water and nonsolvent is taken off the top of extractor 12 and conveyed to distillation unit or still 18 where high boiling fraction 20 is taken off the bottom as a waste stream, 1-hexene and traces of water and nonsolvent stream 22 is taken off the top and returned to process following further treatment, and cyclohexane stream 23 is taken off the side and returned to process. Alternately, with proper operation and plant design, cyclohexane can be taken off the top of still 18 as part of stream 22, thus eliminating stream 23. Stream 24, composed of ethanol and water, is taken off the bottom of extractor 12 and passed to distillation unit 26 where it is separated into an overhead stream 28 composed of ethanol-water azeotrope and a bottom stream 30 of hot water.
Extractive distillation unit 32 is provided for the purpose of recovering water-free ethanol from the ethanol-water azeotrope with the aid of a third hydrocarbon, cyclohexane, in this case. Ethanol-water azeotrope stream 28 is augmented by addition thereto of cyclohexane through line 34 and the combined stream 36 is introduced into extractive distillation column 32 where ethanol is recovered from the bottom as stream 38, which is returned to the process, and where vapor stream 40 is taken off the top and conveyed to condenser 42 where it is converted to a condensate stream 44. The condensate stream is a ternary azeotrope of cyclohexane, ethanol, and water which is recycled to stream 10. Stream 46 of high boilers is taken from the bottom of column 32 as waste. Line 29 is nonexistent when a water-free nonsolvent hydrocarbon is used in the polymer extraction process.